Unless you have an expensive bike, you'll probably spend more on accessories than the initial bike purchase. Budget at least $500 to start. You'll need a helmet, water bottle/cage, CO2 inflator, spare tube, tire levers and saddle bag so you can quickly fix a flat and get underway. Fenders, gloves, lights, rack, trunk bag, and cycling shorts would be my next purchases. Then clipless pedals and shoes. The list goes on and on.